In the papermaking industry, it is generally known that paper to be converted into a consumer product such as paper towels, bath tissue, facial tissue, and the like is initially manufactured and wound into large, round rolls. In many instances, these rolls, commonly known as parent rolls, may be on the order of 10 feet in diameter and 100 inches across and generally comprise a suitable paper that is convolutely wound about a core. Typically, a converting facility will have a sufficient inventory of parent rolls on hand to be able to meet the expected demand for the paper conversion to products such as paper towels and facial tissue as the paper product(s) are being manufactured.
Because of the compressible nature of the paper used to manufacture products like paper towels, bath tissue, facial tissue, and the like, it is quite common for parent rolls to become out-of-round. Not only the soft nature of the paper, but also the physical size of the parent rolls, the length of time during which the parent rolls are stored, how the parent rolls are stored (e.g., on their end or on their side), and the fact that ‘roll grabbers’ used to transport these parent rolls clamp the parent roll generally about the circumference all can contribute to this problem. As a result, by the time many parent rolls are placed on an unwind stand for converting, they have changed from the desired cylindrical shape to an other-than-round (e.g., out-of-round) shape.
In extreme cases, parent rolls can become oblong, assume an ‘egg-like’ shape, or even resemble a flat tire. But, even when the parent roll is only slightly out-of-round, there are considerable problems. In an ideal case, as material is removed from a completely round, convolutely wound parent roll, the feed-rate, web velocity, and tension will generally be consistent. However, process disturbances such as the feed-rate variability, web velocity variability, and tension variability for an out-of-round, convolutely wound parent roll, caused by the shape changes created by the storage and handling of parent rolls, will likely vary the material removal from the ideal web speed of a completely round parent roll depending upon the position and/or radius at the web takeoff point at any moment in time.
If the rotational speed of the parent roll remains substantially constant, the feed-rate, web velocity, and tension of the web material coming off of an out-of-round parent roll will vary during any particular rotational cycle. Naturally, this depends upon the degree to which the parent roll is out-of-round. Since the paper converting equipment downstream of the unwind stand is generally designed to operate based upon the assumption that the feed-rate, web velocity, and tension of web material coming off of a rotating parent roll is generally consistent with the driving speed of the parent roll, web velocity, and/or tension spikes, and/or slackening during the unwinding process can cause significant problems.
While a tension control system is typically associated with the equipment used in a paper converting facility, the rotational speed and the takeoff point radius can be continuously changing in nearly every case. At least to some extent, these process disturbances are unaccounted for by typical tension control systems. It can be dependent upon the degree to which the parent roll is out-of-round and can result in web feed rate variations and corresponding tension spikes and slackening. These problems can be exacerbated by the need for faster unwind speeds to accommodate the need for faster production output.
With an out-of-round parent roll, such process disturbances cause the instantaneous feed-rate, web velocity, and/or tension of the web material to be dependent upon the relationship at any point in time of the radius at the drive point and the radius at the web takeoff point. As previously mentioned, it is known that out-of-round parent rolls may not be perfectly oblong or elliptical but, rather, they may assume a somewhat flattened condition resembling a flat tire, or an oblong or egg-shape, or any other out-of-round shape depending upon many different factors.
Regardless of the exact shape of the parent roll, at least one point in the rotation of the parent roll exists where the feed rate of paper to the line is at a minimum. At this point, the web tension can spike since the feed rate of the web material is at a minimum and is lower than what is expected by the paper converting equipment downstream of the unwind stand. Similarly, there can exist at least one point in the rotation of the parent roll where the feed rate of paper to the line is at a maximum. At this point, the web tension can slacken since the feed rate of the web material can be at a maximum and more than what is expected by the paper converting equipment downstream of the unwind stand. These process disturbances are not conducive to efficiently operating paper converting equipment for manufacturing paper products such as paper towels, bath tissue and the like. A process disturbance, such as a spike in web tension, can even result in a break in the web material requiring a paper converting line to be shut down.
Clearly, there is a need to overcome this problem. Particularly, out-of-round parent rolls create variable web feed rates and corresponding web tension spikes and web tension slackening that have required that the unwind stand and associated paper converting equipment operating downstream thereof be run at a slower speed. In many instances this creates an adverse impact on manufacturing efficiency.
While various efforts have been made in the past to overcome one or more of the foregoing problems with out-of-round parent rolls, there has remained a need to successfully address the problems presented by web feed rate variations and corresponding web tension spikes and web tension slackening.